In the recovery of gold from its mineral sources with which gold is associated, a number of steps and combinations of steps have been proposed to improve the yields, that is, the recoverability of gold.
As the recoverability is a function of the refractoriness of the ore, a number of exploratory attempts have been made to obtain precious metals from the ore under the typical conditions under which, e.g., gold, has been extracted from these ores.
For recovery of gold, placer ores are, of course, the easiest ores to work. On the other end of the spectrum of ores, carbonaceous sulfide ores, that is, ores containing sulfides, e.g., pyrites, arsenopyrites, etc., and both inorganic and organic carbon characterize ores especially refractory for the recovery of precious metals using a typical cyanide process (even though these ores contain fairly high amounts of precious metals, such as gold and silver).
The standard method in the industry for extracting gold from gold bearing ores is cyanidation. This has been the industry's preferred method, such as for the recovery of gold from oxidized ores.
Inasmuch as cyanidation has proven, as shown below, to extract a negligible amount of gold from these refractory carbonaceous sulfide ores, many attempts have been made to improve recovery in the ore treatment, e.g., pretreatment, and the cyanidation step. For example, carbon in the form of 6.times.16 mesh activated coconut shells or extruded peat (Norit 3515) has been added to the leach slurry or solution when cyaniding the ore. Typically this method is denoted as "carbon-in-leach" cyanidation (CIL) or, with minor modifications, "carbon-in-pulp" (CIP) cyanidation.
Still further, this improvement, i.e., carbon-in-leach or carbon-in-pulp cyanidation, has been coupled with various pretreatment procedures to which the ore has been subjected. A great number of these pretreatment procedures have been described in the prior art and alleged as improving the results. A number of these pretreatment steps have been carried out under atmospheric conditions or under pressure in an autoclave.
Some of the shortcomings of these prior art pretreatments have been associated with undue consumption of the materials with which the ore has been treated, or producing unacceptable consequences in subsequent treatment steps. Other shortcomings have been an undue increase in treatment lixiviant consumption, or the leaching time or temperature constraints unacceptable for the efficacious recovery of the precious metals from the ore, and the like.
Nevertheless, some of the autoclave pretreatments in the so-called carbon-in-leach cyanidation have resulted, indeed, in a remarkable improvement when measured against the cyanidation, that is, straight cyanidation or carbon-in-leach cyanidation.
However, the potential or ultimate amount of gold in the ore which could be recovered still has been a goal against which all attempts have been measured. This goal has eluded many attempts, especialy on an industrial scale, and has been an incentive for a number of investigations. Such potentially complete recovery, although alleged to have approached substantially complete exhaustion of gold from the ore, has been mere speculation or economic nonsense. Hence, with respect to the autoclave pretreatment with carbon-in-leach or carbon-in-pulp cyanidation, various pretreatments still have fallen short of a complete exhaustion or substantially complete exhaustion of gold in the ore.
The degree of recoverability of gold is still influenced by and is a function of sulfides, the metal content of the ore associated with sulfides and, more importantly, the carbonaceous compound content of the ore.
Thus, with an increase of sulfide sulfur and organic carbon, all other conditions being equal, the refractoriness of the ore increases.
Refractoriness of ore, by definition, is based on the difficulty of each ore with which it, when treated by simple cyanidation, makes it difficult to extract gold from it or any precious metal recovered with gold.
A considerable effort has been devoted to the recovery of increasingly greater amounts of gold from these refractory ores. Such efforts have been illustrated, for example, in U.S. Pat. No. 4,259,107, and the prior art mentioned therein.
Similarly, U.S. Pat. No. 4,038,362 likewise discloses prior art methods and discusses these methods. This discussion is in the context of the prior attempts which have sought to increase the recovery of gold from organic carbonaceous sulfide ores. Other efforts have been illustrated in U.S. Pat. No. 3,574,600 and 3,639,925.
U.S. Pat. No. 4,289,532 discloses oxygen gas oxidation of carbonaceous gold containing ores in an alkaline medium followed by chlorination of the ore. Use of an alkaline medium has been asserted to be a critical requirement in the process. However, oxidation of sulfidic ores produces acid, and maintaining an alkaline medium requires initial introduction and augmentation with an alkali material so as to maintain this alkaline medium. Costwise and material handling-wise, additional material usage causes the process as disclosed in U.S. Pat. No. 4,289,532 to be less attractive compared to the disclosed process. Although the process disclosed in U.S. Pat. No. 4,289,532 may be favored for alkaline ores, i.e., ores containing dolomite and calcite, for sulfidic-carbonaceous ores the present process is vastly more favorable. (However, it is noted that the present process is less suited for ores which are classified as dolomitic or calcitic ores.)